1. Technical Field
This disclosure pertains generally to agricultural sensors and plant status detection schemes, and more particularly to an apparatus and system of sensors and methods that monitor leaf temperature and corresponding microclimatic conditions in real-time and produce a reliable plant water status indicator. The sensor system can be integrated with a wireless network to continuously monitor the plant water stress status and control an automated variable-rate irrigation management system. It can also be used as a standalone unit to record plant water status continuously.
2. Background Discussion
The aim of irrigation scheduling for crops is to maximize water use efficiency, to maximize yield and to reduce the overall demand on water resources. Current practices of irrigation scheduling followed by growers are mostly based on the condition of the crop and the feel of the soil. Automatic irrigation scheduling techniques developed over the years usually monitor “soil moisture content” or “soil water balance” and uses that information to make decisions regarding irrigation events. Even though the soil monitoring approach to irrigation scheduling has been found to be fairly successful in field crops that have a relatively shallow root zone as compared to tree crops, the approach is problematic because it ignores the physiological responses of the plants to changing environmental conditions. The soil moisture content does not represent the available water to plants, especially tree crops that have a vast root system, as the moisture measurement using a soil moisture sensor indicates the available moisture at the location of the sensor rather than for the whole root zone.
Because plants respond directly to changes in the water status of their tissues, sensing plant water stress has been considered to be a better indicator of irrigation requirements than monitoring soil water content. Various tools have been used to measure plant water stress for irrigation scheduling purposes. Using a pressure chamber is the most common method of measuring stem water potential (SWP) for tree crops. Although mid-day SWP measurements taken by a pressure chamber are considered as the standard method, it is a very time consuming and labor intensive procedure. This makes it an impractical technique for development of an efficient precision irrigation scheduling system that requires frequent and high density spatial SWP data collection.
Thermal sensing is also a promising technique used to sense plant water stress by measuring canopy temperatures using infra-red thermometers (IRT) or thermal cameras. Stomates of any healthy plant that is not under water stress tend to open fully when exposed to sunlight, which in turn increases transpiration. The evaporation of water through the leaf stomata during the transpiration process cools the leaf surface, which is an indicator of the extent of opening or closing of the leaf stomata. Therefore, differences in leaf temperatures compared to ambient temperatures have been studied to determine the water stress level of plants.
Crop water stress index (CWSI) is one of the most commonly used indices used to quantify plant water stress for irrigation management. However, it has been found that it requires extensive measurements in a wide range of vapor pressure deficit (VPD) conditions to obtain meaningful values of CWSI. It can also be sensitive to environmental factors like wind and PAR creating inaccuracies.
Accordingly, there is a need for a system and method for evaluating crop water status that makes taking extensive measurements over a wide range of VPD for calculating meaningful values of stress indices (e.g. CWSI) inexpensive, quick and easy.